The invention relates generally to subsea drilling risers. More particularly, the invention relates to breakaway connections for subsea risers and intervention devices and Methods disconnection of subsea risers.
In offshore drilling operations a riser is employed to flow mud returns from a borehole extending from the sea floor to a drilling vessel or rig disposed at the sea surface. The upper end of the riser is connected to the drilling vessel and the lower end of the riser is connected to a lower marine riser package (LMRP) that is mounted to a subsea blowout preventer (BOP) secured to a wellhead at the sea floor. The upper end of the riser is effectively fixed to the drilling vessel and the lower end of the riser is effectively fixed to the sea floor, and the riser is typically placed in tension therebetween.
Although drilling vessels employ dynamic positioning (DP) systems to maintain their positions relative to the corresponding subsea wellheads, drilling vessels do experience heave and lateral movements in response to wind and waves at the surface. Consequently, risers often experience variations in tensile loads, bending moments, and torsional loads during offshore drilling operations. In an extreme scenario, a failure or malfunction of a DP system may cause a drilling vessel to drift off location, thereby applying drastic increases in the tensile loads applied to the associated riser. If the tensile, bending, and/or torsional load capacity of such a riser is exceeded, it may break. Upon breaking, the upper portion of the riser attached to the drilling vessel will be supported by and suspended from the drilling vessel, however, the lower portion of the riser attached to the LMRP is unsupported, and thus, will fall down to the sea floor. The fallen lower portion can damage subsea equipment such as the LMRP or BOP, or wellhead, as well as create a tangled mess of debris around the LMRP, BOP, and wellhead. Damage to the subsea equipment and tangled debris may undesirably obstruct and/or limit subsequent remedial operations.
Failure analysis of several conventional riser systems suggests the weak links in riser systems is often at the connection between the upper end of the riser and the drilling vessel. A break at this point of a riser is particularly problematic as the unsupported lower portion of the riser that ails to the sea floor represents almost 100% of the total length of the riser. The long length of the separated lower portion and associate mass enhances the likelihood of damaging subsea equipment and creating obstacles that may interfere with subsequent remedial operations.